The present invention relates to phase shift circuits, and in particular to 90 degree adjustable phase shift circuits for sampling an eye pattern of a received signal to recover data from transmission lines.
In data transmission systems, after a clock signal is recovered, it is desirable to shift the clock signal by 90 degrees so that data can be sampled with the clock edge at the middle of the eye pattern. This shifting is needed in order to reduce the bit error rate and is used in both line receivers and repeaters.
A typical method for producing a phase shift is to use a capacitor to delay the rise time of the signal enough to produce the desired phase shift. An example of this is shown in FIG. 1 where a Vin signal 10 is presented to a capacitive circuit. For a first capacitive value, a signal 12 is produced which crosses a threshold value 14 a period of time N after the input signal to produce an output signal (Vout1) 16 which is phase shifted by N. By using the larger capacitance, a signal 18 is produced which crosses threshold 14 at a later time M to produce a phase shifted signal (Vout2) 20 which is shifted by an amount M.
FIG. 2 is a block diagram of a typical clock recovery circuit. A transmission line 43 is coupled into a transformer 44 and then provided to an equalizer and preamplifier 45. The signal is then provided to a threshold detector 47. For AMI bipolar line code transmission, a full wave rectifier is used with a threshold detector to square the received signal. For other types of transmissions, another circuit could be used which raises the received signal to a power of N depending upon the type of transmission coding.
A tank circuit 46, tuned to the clock frequency, is excited by the full wave rectifier output and produces a sine wave signal whose amplitude is data pattern dependent. A limiter 48 converts the sine wave signal into a square wave signal. This square wave is the recovered clock signal. Phase shifter 52 then shifts the clock signal so that the edge of the clock will fall in the middle of the eye pattern. In the example shown, the phase shifter is a simple RLC network. The inductor is optional and is used to emphasize the edges of the waveform. A limiter 54 is used to clip the signal and provide the shifted clock for data recovery.
Because the clock recovery circuitry itself will introduce phase shifts into the signal, some designers adjust tank circuit 46 to tune the circuit to compensate for this internal phase shift in receiver/repeater production. Unfortunately, this will degradate the signal/noise and jitter performance of the circuit.
Another version of phase shifter 52 is shown in FIG. 3. An input signal is applied across terminals 22, 24 to differential transistors 26, 28. These transistors are coupled to a current source 30 and to a positive voltage VCC through resistors 32 and 34. A capacitor 36 is used to provide the output signal shown in FIG. 1 across output terminals 38 and 40. The output signal is an RC exponential signal which is provided to limiter 54 of FIG. 2.
In integrated repeaters and receivers, an external capacitor is typically used for capacitor 36 because of the large value required to obtain a 90 degree phase shift. By using a large time constant (e.g., by using a large capacitance), signal 18 becomes almost linear in the region of interest in FIG. 1, thus giving a 90 degree phase shift. In addition, by making the capacitor external, the system can be adjusted for the frequency of the transmission. For instance, in Europe, PCM systems use 2 megabit/second data rates as opposed to the U.S. rate of 1.544 megabit/second. The external capacitor must be changed to provide the required phase shift at each frequency which may be used.
Other methods not using an external capacitor are believed to use an automatic gain control (AGC) control loop which converts the input signal to a triangular wave and picks a slicing point in the middle of the triangular wave for reproducing a phase shifted square wave. This approach complicates the circuitry and requires more silicon.